Aggregation-Induced Emission Poly(meth)acrylates for Photopatterning via Wavelength-Dependent Visible-Light-Regulated Controlled Radical Polymerization in Batch and Flow Conditions

A robust wavelength-dependent visible-light-regulated reversible-deactivation radical polymerization protocol is first reported for the batch preparation of >20 aggregation-induced emission (AIE)-active polyacrylates and polymethacrylates. The resulting polymers possess narrow molar mass distributions (Đ ≈ 1.09–1.25) and high end-group fidelity at high monomer conversions (mostly >95%). This demonstrated control provides facile access to the in situ generation of complex sequence-defined tetrablock copolymers in one reactor, even while chain extending from less reactive monomers. Polymerizations can be successfully carried out under various irradiation conditions, including using UV, blue, green, and red LED light with more disperse polymers obtained at the longer, less energetic, wavelengths. We observe a red shift and wavelength dependence for the most efficient polymerization using LED illumination in a polymerization reaction. We find that the absorption of the copper(II) complex is not a reliable guide to reaction conditions. Moreover, the reported protocol is readily translated to a flow setup. The prepared AIE-active polymers are demonstrated to exhibit good photopatterning, making them promising materials for applications in advanced optoelectronic devices.


■ INTRODUCTION
Luminescent materials have fundamental and technological significance, attracting both academic and commercial interest. Until the aggregation-induced emission (AIE) concept was coined, much effort was put toward applying aggregationcaused quenching effects of conventional luminophores with large conjugated coplanar molecular configurations. 1 Comparatively, AIE-active molecules often possess twisted propeller-shaped structures, resulting in hindered intramolecular motion when aggregated. Accordingly, the emission of an AIEgen can be turned on in an aggregated state. 2 An increasing array of AIE-containing polymeric systems have been explored via various synthetic methodologies, such as ring opening polymerization, 3 polycoupling, 4 click polymerization, 5−7 and multicomponent polymerization. 8−10 The resulting AIE-active polymers can exhibit excellent processability, stability, and biocompatibility, allowing for practical applications in diverse areas including optoelectronics, sensing, imaging, and biological therapy. 11−13 Nevertheless, there is limited work concerning a facile synthesis of well-defined and narrow-disperse AIE polymers. Note that dispersity can be a key factor in polymer design and can impact material properties, for example, in self-assembly and mechanical performance. 14,15 Controlled dispersity is often reported to be beneficial for systematic investigation and prediction of the structure−property relationships of macromolecules. 16−18 Reversible-deactivation radical polymerization (RDRP), or controlled/living radical polymerization, including nitroxide mediated polymerization (NMP), reversible addition-fragmentation chain transfer (RAFT), and atom transfer radical polymerization (ATRP) polymerization, has revolutionized polymer science, enabling good control over diverse molecular topology, molecular weight, and chain length distributions. 16,19,20 Other outstanding benefits of RDRP include the ability to perform chain extension with high tolerance to varying reaction conditions 21,22 and chemical functionalities, 23−25 collectively contributing to its robustness and versatility with respect to many vinyl monomers. 16,19,26−29 In particular, a broad interest in light-mediated RDRP arises as it allows for spatial and temporal control over reaction kinetics, monomer sequences, and compositions via external regulation of the reversible activation−deactivation equilibrium using a renewable source of energy. 30−36 Photochemistry can also offer sustainable reaction conditions in a move to supply energy by either photons or electrons in chemical synthesis. As such, functional materials are accessible employing RDRP with applications including nanotechnology 37,38 and therapeutics. 20,39 There are a few examples describing the synthesis of AIE polymers via RDRP via thermal and non-photochemical processes. Tang et al. 40 synthesized and screened a group of tetraphenylethene (TPE)-containing dithiocarbamates as RAFT agents and implemented them for the synthesis of AIE polymers with a range of functionalities. Furthermore, the fluorescence was exploited to visualize the polymerization process in real time. Bao et al. 41 prepared a library of polystyrenes with varied molar mass initiated by a bifunctional naphthalene diimide (NDI)-based AIE-active ATRP initiator. Note that benefiting from the excellent controllability of ATRP, the emission of the resulting polymers in the solid state could be finely tuned by precisely manipulating the monomer substituent variations, end-group transformation, and polymer chain length. There are still opportunities for the expansion of RDRP, in particular light-mediated protocols, for the preparation of diverse AIE polymeric materials with narrow chain length distributions, controlled chain length, and welldefined structures. 42 Due to the differing solubilities of monomers and polymers and the different stabilities of radicals derived from monomers (e.g., acrylic, methacrylic, and styrenic), there is no one set of standard conditions for copper-mediated RDRP, or ATRP. For example, Cu(0) wire-induced RDRP is usually carried out in quite polar aprotic solvents such as DMSO, IPA, etc., which is very suitable for monomers and polymers soluble in these polar solvents and it is noted that these solvents also promote disproportionation of Cu(I) in the presence of aliphatic tertiary amine ligands such as Me 6 TREN and PMDETA. Polymerization in water of water-soluble monomers to watersoluble polymers is efficiently carried out, allowing Cu(I) to fully and rapidly disproportionate to Cu(0) and Cu(II) prior to addition of monomers and initiators. These aqueous conditions are very suitable for many acrylamides and watersoluble acrylates, which have high rates of polymerization. In our previous work, we have exploited the AIEgen-containing initiator tetraphenylethene bromoisobutyrate (TPEBIB) to generate a range of TPE-terminated polyacrylates and polyacrylamides with AIE properties via both Cu(0) wiremediated and aqueous RDRP, respectively. 43,44 AIE-terminated polymers have been reported widely as fluorescence probes for pH, 45 temperature, 46 and in bio-related areas. 47 Among them, TPE-terminated poly(meth)acrylates were reported to have good film-forming ability and thermal stabilities, which could be used as potential fluorescence sensors for explosive detections, 48−50 solution viscosity modifiers, 51 and cell imaging. 43 Effective controlled RDRP can also be carried out via the photoreduction of Cu(II) in the presence of a greater than two-fold excess of appropriate tertiary amine ligands, such as Me 6 TREN. 52 In our previous work, we had hypothesized that irradiation occurs into both a free ligand absorbance and the alkyl bromide initiator and not into the copper(II) complex. 53,54 Recently, Barner-Kowollik et al. probed the wavelength dependence of the photochemically induced copper-mediated polymerization of methyl acrylate between 305 and 550 nm, reporting the reactivities and comparing monomer conversion, molecular weights, and dispersity with the absorption spectrum of the copper(II) complex. 55 For this, a specialized wavelength-tunable nanosecond pulsed laser polymerization (PLP) method was used to produce the socalled action plots at a constant photon flux while varying the excitation wavelength. Both the molecular weight and molecular weight distribution showed a wavelength dependence, illustrating that a choice over wavelength is desirable. This later work also reports that the absorption spectrum of the copper(II) complex does not represent a robust guide for monomer-to-polymer formation, illustrated with an observed red shift away from the absorbance maxima to give the most effective polymerization.
Herein, we combined our interest in the design of AIEfunctional polymer materials with our continued investigations into light-induced RDRP by presenting the synthesis of a library of AIE-active polymers using visible-light-regulated RDRP in both batch and flow reactors (Scheme 1). Specifically, a total of 22 different monomers with diverse architectures were polymerized using a 1,1,2,2-tetraphenylethene functional ethyl α-bromoisobutyrate (TPEBIB) initiator. We further aimed to investigate the wavelength dependence of the TPEBIB-initiated polymerization of methyl acrylate in terms of both control and rate. For this, we used commercially Scheme 1. Reaction Scheme for the Polymerization of a Library of (Meth)acrylates with the Initiator TPEBIB via Visible-Light-Mediated Cu-RDRP in Both Batch and Flow Reactors available LED arrays in batch instead of relatively expensive, and generally inaccessible, PLP systems, thereby making it more transferable to many synthetic laboratories. We were also interested in how normal batch-type reactions would translate into a flow system using similar photochemical stimulation. This also allowed us to move away from quite hydrophilic polymers of our previous studies to many more relatively hydrophobic monomers/polymers, which form the majority of this type of polyacrylate and polymethacrylate used in bulk and film applications.

■ RESULTS AND DISCUSSION
Initially, the photoinduced homopolymerization of methyl acrylate (MA) (targeted DP n = 100) was attempted using a standard method developed by our group 54 using a UV curing box (broad band with λ max ≈ 360 nm, Figure 1) with the UV emission ranging from <200 nm to well into the visible region. A sample after 20 h showed 41% monomer conversion by 1 H NMR with the polymer having a dispersity of 1.43 ( Figure  2A). A 96-well LED reactor, which is available from a commercial supplier with a wide range of single wavelengths generated from LEDs ( Figure 1) in a simple interchangeable way, was also employed for a series of polymerizations using TPEBIB as an initiator ( Figure 2A and Table S1). The setup of the LED reactor is shown in Figure 3 and Figure S1. Monomer conversion at λ max = 365 nm reached 31% after 6.5 h, however, with a broader dispersity = 2.18. A 99% monomer conversion with a product with a significantly lower dispersity = 1.13 was obtained at a lower energy (λ max = 405 nm). However, a small higher-molecular-weight shoulder in the mass distribution was also observed (Figure 2A). Further increasing the wavelength toward the more visible light part of the spectrum resulted in 96% conversion with Đ ≤ 1.10 and a very monomodal mass distributions in 4−15 h under blue (λ max = 470 nm), green (λ max = 527 nm), or even red (λ max = 630 nm) LED ( Figure  2A). The blue LED (λ max = 470 nm) was selected to perform further polymerizations owing to the faster reaction rate as well as maintaining excellent control over polymerization. This is in broad agreement with Barner-Kowollik and co-workers in that the absorption of the complex does not correlate with the most efficient reaction conditions and a red shift is observed from the maximum absorbance for the best reaction conditions ( Figure 4).
Interestingly, even with irradiation at a lower energy, relative to any apparent significant absorption, we observe high monomer conversions over these prolonged reaction conditions with good, if not better, control over molecular weight and dispersity compared to a more intense higher energy UV    Figure 4A). This is in broad agreement to that reported by Barner-Kowollik and co-workers with our study demonstrating that the molar extinction coefficient of the complex is not a reliable indication to select the optimum irradiation wavelength. We note that the free ligand does have a tail of the absorption band well into the visible region in a seemingly exponential decay and does not reach zero absorption ( Figure  4B). Thus, this is consistent with our original observation that the important photon absorption is into the free ligand followed by energy transfer to the complexed ligand. 53 We do note that a major difference between this data and the type reported by Barner-Kowollik et al. is that in their case, they take care to use identical amounts of photons, which could not be achieved with our commercial LED setup. Furthermore, we prioritized to take the polymerization reactions to high monomer conversion rather than time limit stopping reactions at low-to-moderate conversion. From the conversion plots as a function of irradiation wavelength, we identified the blue lightinduced process (λ = 470 nm) as providing quite intriguing polymerization conditions given the minimal spectral overlap with the copper complex.
The molecular characteristics and in general the quality of the obtained polymers are highly dependent on the excitedstate dynamics, the redox behavior, and the different species that can be generated upon UV irradiation. Since the AIE initiator has a significant absorption across the UV spectral range, it could thus potentially interfere with the photopolymerization. The absorption spectrum of TPEBIB and spectral characterizations of different LEDs are shown in Figure 1. There is an overlap between the absorption spectrum of the initiator and the used UV LEDs (λ max = 365 and 405 nm), indicating that the absorption by the initiator seems to be detrimental to the polymerization efficiency. To test this hypothesis, similar reactions were carried out using EBIB as the initiator, having very little to no absorption above 300 nm ( Figure 5). As a result, polymers with low dispersity at high conversions were all accessible regardless of the chosen wavelength of the LED or the light intensity used, with a 94−99% monomer conversion and Đ = 1.08−1.24 depending on the exact conditions (Table S2). Thus, loss of control is observed when the initiator has more significant absorption of the incident light.  Subsequently, the effect of the intensity of the blue LED irradiation was further investigated. Lumidox II devices are calibrated with five discrete linearly stepped output stages. Stage 1 output has the least radiometric power, while stage 5 output is the highest. The details of the power and irradiance used are listed in Table S3. The kinetics of the polymerization of MA is demonstrated in Figure 2B, with five varying stages of applied intensity. The reactions were conducted using identical settings, with the temperature being 5°C in the external cooling circulator. Despite the same targeted temperature, there is still a significant actual temperature difference of the reactions. Reaction temperatures were measured as 26, 40, 54, 66, and 90°C, respectively, when the intensity was increased from stage 1/5 to 5/5. This results from the inability of the thermal deck attached to the lamp to not cool the reaction efficiently due to the intense radiation from the LED array. Nonetheless, good linear first-order kinetics throughout the reaction was observed for the two lowest intensities (stages 1/ 5 and 2/5). However, a further increase in the intensity resulted in a nonlinear relationship between ln[M]/[M 0 ] and the reaction time, especially when the conversion of MA was >85%, suggesting the lack of control at a higher monomer conversion. Additionally, it is much more challenging to reach high conversions (>90%) if the reaction temperature is too high ( Figure 2B, stage 5/5), attributed to increased termination occurring. This was further confirmed by the polymerization results conducted under the same light intensity but varying temperatures ( Figure 2C). Considering both the good linear control and reasonable reaction rate, stage 2/5 was chosen for future polymerization reactions. The effect of catalyst concentration was also investigated ( Figure S3) with the highest conversion (96%) and lowest Đ (1.09) achieved in 4 h with CuBr 2 :Me 6 Tren:initiator = 0.02:0.12:1.
Using identical optimized conditions, a library of both welldefined hydrophilic and hydrophobic poly(meth)acrylates from >20 monomers was successfully synthesized with targeted M n ≈ 9000 (Figure 6A−V and Table S4). These monomers include linear and branched (meth)acrylates with varying alkyl length (C1, C2, C4, C6, C8, C12, or C18). Additionally, fluorine-containing poly(trifluoroethyl acrylate) (PTFEA) was also successfully prepared ( Figure 6L). In most cases, high conversions were reached (≥95%) with quite low dispersities ranging from 1.09 to 1.25. Significantly, the negligible deviation between the theoretical (M n,th ) and experimental molecular weights (M n,NMR and M n,GPC ) (Table S4) as well as symmetrical monomodal SEC traces without tailing demonstrated excellent control.
Similarly, a series of polymethacrylates were prepared employing the same solvents used for polyacrylates with the same side chains. Initially, the identical conditions were used for polymerization of methyl methacrylate (MMA), furnishing a polymer with Đ = 1.39 at 83% monomer conversion (Table  S7). To accelerate the reaction rate, the catalyst amount was tripled to CuBr 2 :Me 6 Tren:initiator = 0.06:0.36:1, which leads to a near full monomer conversion (98%) and a decreased Đ = 1.26. By replacing Me 6 Tren with N,N,N,N,N- Table S4).
We subsequently investigated the potential of this system in maintaining good control across a wide range of molar masses. Polymerizations of MA targeting degrees of polymerization (DP n ) from 100 to 400 were performed, resulting in high   Figure 6W and Table S8).
In situ chain extension is widely employed to both verify high end-group fidelity and allow easy access to multiblock functional copolymers with no need for purification between the iterative monomer additions. Chain extension from P(PEGA 480 ) was attempted, employing identical previously developed conditions (Table S9). After 8 h, a near full monomer conversion (98%) was attained (Đ = 1.16) and a deoxygenated solution of a second block (EGA) in DMSO (1/ 1) and the catalyst were subsequently injected into the reaction mixture. In a similar manner, following several monomer additions, an amphiphilic tetrablock (TPE-P(PEGA 480 ) 5 -b-PEGA 25 -b-PMA 15 -b-PEA 15 ) was achieved (Đ = 1.23, M n = 7000) ( Figure 6X). Notably, chain extensions from the less active monomers were also attempted ( Figure 6Y and Table  S10). A tetrablock, TPE-PLA 8 -b-PHA 10 -b-PBzA 10 -b-PtBA 10 , was also prepared with the first two blocks reaching full conversions followed by 93% monomer conversions for the last two monomer additions. Although the dispersity tended to slightly increase upon the addition of the subsequent block, the obtained tetrablock still had a reasonable Đ = 1.28. Collectively, SEC analysis revealed monomodal peaks with a shift of the mass distribution to higher molecular weights upon the next block addition while maintaining low dispersities, thus suggesting good end-group fidelity at high monomer conversions.
Temporal control of this blue LED-mediated Cu-RDRP system was examined with the polymerization of MA in DMSO ( Figure S4). An observable yet reproducible amount of polymer growth during the "off" cycles evidenced non-ideal temporal control, which agrees with other photo Cu-RDRP systems. 29,54,56 The unexpected chain growth in the dark was attributed to the increased lifetime of residual activator after initial photoactivation. 57 It was also reported that better temporal control could be achieved by decreasing the catalyst amount. 57 There are advantages in using flow chemistry and especially when considering sustainability aspects of production. Thus, following our batch syntheses, we investigated this protocol using polymerization in a plug flow system using the same LED

Macromolecules pubs.acs.org/Macromolecules
Article arrays in a commercially available flow reactor designed specifically for these arrays ( Figure 3B and more details in Figure S2). The exposure volumes were set to 1.67 mL in total using 3.2 mm outer diameter (OD) with a 1.6 mm inner diameter (ID) tubing. MA, PEGA 480 , and ODMA were chosen for this comparison (Table 1). Competitive control and especially narrow dispersity were observed in the plug flow reactor under the same conditions (Figure 7). By comparing the results in batch and continuous flow reactions, polymers with similar molecular characteristics were obtained, with a slightly lower conversion in the case of flow reactions, which is attributed to the process implemented without stirring. All synthesized TPE-terminated polymers were characterized by NMR ( Figures S5−S8). The TPE group incorporation into the polymers was exhibited by the signals δ = 7.15−6.77 ppm in 1 H NMR spectrum (Figures S5 and S6) and δ = 150−120 ppm of 13 C NMR spectrum ( Figures S7 and S8). Additionally, the characteristic peaks were clearly assigned. Subsequently, some photophysical properties of the polymers were investigated to assess the functionality of the AIE-containing initiator post-polymerization. The AIE behaviors were examined in both THF/water mixtures and in the solid state (Figure 8 and Figure S9). All the polymers exhibited typical AIE properties due to the presence of the TPE moiety. For example, TPE-PMA 100 emitted weakly when solubilized in pure THF. Upon gradual addition of water (a poor solvent for PMA) to the THF solution, the luminescence was slowly enhanced before the water fraction (f w ) reached 80 vol %. Further increasing f w to 98 vol % contributed to the strongly enhanced emission ( Figure 8A). Notably, the photoluminescence intensity of TPE-PMA 100 in 98% THF/water mixture is >90-fold higher than that in pure THF (I 0 ) ( Figure 8B). In a similar fashion, all other TPE-terminated polymers were demonstrated to be AIE-active. In addition, the solid state of these polymers was also measured to be emissive ( Figure 8C). This is ascribed to the restriction of intramolecular motions of the TPE moiety in the aggregated state, which blocks the nonradiative decay pathway of the excited state to turn on the emission of the polymer. 58 Given the efficient emission of the TPE-terminated polymers in both aggregated and solid states, their potential application in photopatterning was explored. A thin TPE-PMMA 100 film was fabricated by spin-coating followed by exposure to UV light (λ = 330−380 nm) in air for 20 min at ambient temperature through a negative copper photomask. A two-dimensional fluorescent photopattern was readily generated ( Figure 8D). The unexposed squares endured bright fluorescence, whereas the emission of the exposed regions was quenched, possibly resulting from the photooxidation process, 59 giving a turn-off-type 2D photopattern. Significantly, the generation of such a fluorescent pattern by a photolithography technique demonstrated that these synthesized TPE-contain-ing polymers are promising materials regarding optoelectronic applications.

■ CONCLUSIONS
Thus, we have demonstrated the wavelength dependence of photoinduced Cu(II) reduction for controlled radical polymerization with simple commercially available LED systems. We find broad agreement with Barner-Kowollik et al. that the

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Article absorption of the copper(II) complex is not a reliable guide to reaction conditions using a commercial system designed to be utilized for routine synthesis. A red shift away from the absorption maximum is observed in accordance with the results achieved using a pulsed laser irradiation. Optimized conditions utilized visible light to give a library of AIE-active hydrophobic (meth)acrylates with side chains containing linear and branched alkyl groups as well as fluorine. It proved important to avoid absorption of the incident light by the initiator to achieve the best control of molecular weight and dispersity. The well-defined polymers and in situ generation of sequential multiblock copolymers show good targeted molecular weights, narrow chain length distributions, and high end-group fidelity at high monomer conversions, allowing for the extension of copper-mediated RDRP to give AIEgencontaining polymers. This strategy is compatible with both batch and plug flow reactors. Moreover, the TPE-terminated polymers exhibited excellent performance in photopatterning, enabling their potential applications in optoelectronics and other film-and bulk-based applications.
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